This invention relates to a valve for use in industrial gas process sYstems, and, in particular, to a main steam stop valve by-pass valve for steam turbine applications.
In the operation of steam turbines in commercial power plants, stop valve by-pass valves (SVBV) play an important role in turbine start-up. During start-up, the gas stream is throttled and controlled at the SVBV only. As a result, start-up and initial loading of the steam turbine may be accomplished with all control valves wide open, resulting in uniform steam flow and warming to all turbine shell passages. The SVBV itself is typically mounted inside a larger master valve assembly, such as a stop valve assembly, wherein the SVBV functions as the valve disc. For purposes of illustration, the discussion which follows will focus on the use of an SVBV in a stop valve assembly. The SVBV performs its by-pass function when the stop valve is closed. Thus, the SVBV, as its name implies, functions as a valve within a valve.
Typically, an SVBV is comprised of a number of components, the largest of which is the stop valve disc. The master valve disc is that part of the SVBV which seats in the stop valve assembly, and includes the outlet ports of the SVBV. Inside the SVBV, a by-pass disc is mounted on a stem which is slidably inserted in the center of the master valve disc, and functions to open or close the SVBV. A valve cap with inlet ports is mounted on the top of the stop valve disc. When the by-pass disc is moved down to seat in a first position in the stop valve disc, the steam flow is restricted or stopped. As the by-pass disc is moved upward, an annulus is created between the by-pass disc and the stop valve disc and steam flows through the SVBV. At a point maximum upward travel, the by-pass disc seats in a second position in the valve cap, and the steam annulus within the stop valve disc is wide open to the inlet and outlet ports. Thus, steam or gas flows freely through the inlet ports and annulus to the outlet ports in the stop valve disc. U.S. Pat. No. 3,013,767 is typical of such components present in prior art valve assemblies.
Problems arise in the operation of steam turbines, as in the operation of other industrial gas process systems, when solid particles carried by the gas stream impinge upon and erode pipes, valves and other process equipment. In particular, steam carries microscopic oxide particles that spall from high temperature boiler tubes, steam headers, and steam piping. High velocity flow and changes of direction cause the particles to impact on stationary nozzles, rotating blades, and valve surfaces, causing erosion. The rate of solid particle erosion is dependent in part on the size, velocity and angle of impact of solid particles, and on the number and duration of turbine start-ups. The central role of the SVBV in turbine start-up subjects the by-pass disc to significant erosion, requiring its replacement at regular intervals to avoid disc failure and forced outages. Solid particle erosion is a long-standing, industry-wide problem with significant economic impact upon operation and maintenance costs, as well as availability and performance, of the nation's power plants.
Thus, reduction or elimination of solid particle erosion of SVBVs in steam turbine applications, as well as in similarly demanding industrial gas process environments, is desirable to decrease operation and maintenance costs and improve system performance.